Pulverizers
Pulverized coal (PC) is the dominant method of firing coal in modern utility boilers, pulverizers are used to grind coal into a fine powder with 70 percent of the ground coal passing through a 200-mesh screen for typical pc applications utility systems in operation today are direct-fired with the coal ground in the pulverizer directly transported to the tamper with a portion of the combustion air (primary air). pulverizer models range in total capacity from 1.5 to 115 tons/h of coal(0.4 to 29 kg/s) with a typical pulverizer shown in FIG.1 . The primary air entering the pulverizer is preheated to temperatures as high as 700℉(371℃) to dry the moisture in the coal and provide stable efficient combustion. the inter temperature is controlled to keep the pulverizer outlet temperature to a specified range which is largely determined by the volatile matter content is the coal .coal with low volatile matter content may require higher temperatures to ensure stable combustion. The typical pulverizer outlet temperature is 150℉(66℃). Table 1 provides outlet temperature ranges for a variety of coal types.
Table 1.Typical Pulverizer Outlet Temperatures
FIG.1.Slow-speed pulverizer,roller-and-race type
FIG.2.Medium-speed pulverizer,boll-and-race type
The three principal types of pulverizers may be classified by their speed low (<75 r/min). Medium ( 75 to 250 r / min). And high ( > 250 r/min). FIG.1 shows a slow-speed. Roller-and-race type of pulverizer. Medium-speed pulverizers are generally either the ball-and-race type (FIG.2) or the bowl-and-roller type (FIG.3).all three of these pulverizers are referred to as vertical spindle pulverizers. High-speed pulverizers are usually of the attrition type. Shown in FIG.4. Some old boilers are equipped with ball-and-tube coal pulverization equipment. In which the coal is ground in a horizontal rotating cylinder that is partially filled with small-diameter tumbling balls. In all pulverizers. Provision is made to remove and dispose of pyrite and other mineral matter from the coal which is hard to grind and which has no material heating value.
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